The invention relates to a circuit arrangement comprising a DC/AC converter for operating a discharge lamp, which DC/AC converter is provided with:
input terminals for connection of the DC/AC converter to a DC voltage source, PA1 a switching circuit connected to the input terminals and provided with at least one switching element, which switching element has a control electrode and a main electrode, which switching element has a control circuit between the main electrode and the control electrode with a series arrangement of means for generating a control signal for the switching element and of first decoupling capacitive means, and which switching element is conductive when a voltage is present between the control electrode and the main electrode with a first polarity and with a value which exceeds a threshold value, PA1 a load branch comprising at least inductive means and output terminals for connection of the discharge lamp, which load branch is supplied via the switching circuit.
Such a circuit arrangement is known from U.S. Pat. No. 4,748,383. The switching circuit A in the known circuit arrangement comprises a further switching element in series with the switching element mentioned above, and a primary winding of a transformer is included in the load branch. Means for generating a control signal are formed by a secondary winding of the transformer. The switching element is periodically switched alternately into a conducting and a non-conducting state by means of the control signal. As long as the voltage between the control electrode and the main electrode of the switching element has a value which differs little from the threshold value, however, the switching element is in a transitional state in which high losses occur in the switching element. It is accordingly desirable that passing of the threshold value, and thus switching between the conducting and the non-conducting state, should take place as quickly as possible. It is also desirable, to reduce the switching losses, that the duty cycle, i.e. the time fraction in which the switching element is in the conducting state, should be comparatively short, for example 30%. An increase in the amplitude of the control signal does cause a quicker passage of the threshold value, but it also causes the duty cycle to increase.